T-2

Frontiers of Research in Gamma-Ray Bursts
ガンマ線バーストの研究最前線
N. Kawai
河合誠之
Department of Physics, Tokyo Institute of Technology,
2-12-1 Ookayama, Meguro-ku, Tokyo 152-8551, Japan
東京工業大学理工学研究科基礎物理学専攻，〒152-8551 東京都目黒区大岡山2-12-1
Gamma-ray bursts (GRBs) are the most energetic explosions in the Universe. Their origin and emission
mechanism are at last starting to be revealed with the coordination of astronomy satellites andground-based observations. Majority of GRBs are now believed to be associated with death of massivestars. Their prompt emission, and their afterglows have been successfully explained by a synchrotron
emission from relativistic shocks.

1. Introduction
Gamma-ray bursts (GRBs) are short episodes of
explosive gamma-ray emission from random
positions in the sky, which typically last for tens ofseconds or less.
Since their serendipitous
discovery in 1960's by nuclear test surveillancesatellites [1], their origin had been a mystery for 30years. Even their distance was unknown.
The discovery of their afterglows in X-ray andoptical bands in 1997 [2,3], at last, convincingly
showed that GRBs are the most violent explosion in
the Universe that the mankind knows (except forthe Big Bang) at cosmological distances with atypical redshift of >0.5.
The only plausible
candidates capable of releasing such large energies
are the core-collapse of a star with tens of solar
masses, and merger of relativistic compact stars (i.e.
neutron stars or black holes), either of which results
in a release of huge gravitational binding energy.
In a widely accepted scenario, accretion of stellar
remnants to the newly born black hole causes
collimated, repeated ejection of relativistic shells(“jets”) with a Lorentz factor of a few hundreds.
The collision of the shells forms shocks.
Synchrotron radiation from the shocks isrelativistically beamed and observed as a
gamma-ray burst. The relativistic shells propagate
into the interstellar medium to form external shocks,
where the accelerated particles emit afterglows in a
broad band (radio to X-rays) that decay with time
following a power-law with indices of 1~2 [4,5].
The fireball model successfully explained various
properties of gamma-ray bursts (prompt emission)and their afterglows. However, the fundamentalquestions remained to be answered. What class ofobjects produce the explosion? When in thehistory of the Universe did GRBs started to occur?
What influence GRB had on the their environment

and the evolution of the Universe? How suchhuge energies can be released? How can suchhighly relativistic jets be formed and collimated?
What can we learn from GRBs about the history of
the early Universe...
We are now beginning to find handles to answer
these questions. In this review I highlight a fewtopics in the study of gamma-ray bursts in thelatest years.
2. The HETE-2 Satellite
For the study of GRBs, detection of theafterglows and the study of their prompt burstemission are both important. The High Energy
Transient Explorer 2 (HETE-2) is the first satellite
designed specifically for the study of GRBs. It
was launched in October 2000, and is currently
localizing gamma-ray bursts (GRBs) at a rate of~20 per year, some of them in real time. The
primary goals of the HETE-2 mission are thebroadband observation of the burst emission ofGRBs, and the prompt distribution of precise GRB
coordinates to the astronomical community for
immediate follow-up observations [6]. To achievethese goals, HETE-2 is equipped with three sets of
gamma-ray and X-ray detectors, which share acommon field of view of ~1.5 steradians, and,together, measure the light curves and energy
spectra in the photon energy range of 2 keV to over
400 keV.
3. Origin of GRBs
There has been increasing circumstantial andtantalizing evidence in the last few years that GRBs
are associated with core collapse supernovae (SNe).
In particular, detection of a powerful type Ic
supernova SN1998bw positionally and temporally
coincident with GRB980425 have been often

discussed as evidence for the GRB-SN association[7]. However, the low redshift of SN1998bw(z~0.008) implied that the gamma-ray luminosity ofGRB980425 was ~104 times fainter than any other
GRB observed to date. If their association is real,
GRB980425 may not be a typical cosmological
gamma-ray burst.
The detection and localization of GRB 030329
by HETE-2 [8] led to a dramatic confirmation ofthe GRB-SN connection.
GRB 030329 wasamong the brightest 1% of GRBs ever seen.
Given that GRBs typically occur at z~1—2, theprobability that the source of an observed burst
should be as close as GRB 030329 (z=0.167) is one
in several thousand.
The fact that GRB 030329 was very brightspurred the astronomical community — bothamateurs and professionals — to make anunprecedented number of observations of the
optical afterglow of this event. More than 170
GCN Circulars have appeared so far, reporting
optical, IR, and radio observations of the afterglow.
About ten days after the burst, the spectral
signature of an energetic Type Ic supernova
emerged [9] on top of the usual GRB afterglow
continuum of non-thermal synchrotron emission.
The underlying supernova has been designated SN
2003dh. The spectrum of SN 2003dh is strikingly
similar to that of the Type Ic supernova SN 1998bw,
which was putatively associated with GRB980425.
The clear detection of SN 2003dh in the afterglow
of GRB 030329 confirmed decisively theconnection between GRBs and core collapse
supernovae. We note, however, that evidence for
association with supernovae has been found onlywith the “long soft” GRBs. We have no clue for“short hard” GRBs, which constitutes ~20% of the
GRB population and for which no opticalafterglows have been found.
.
4. Spectral Energy Peak and Radiated Energy
HETE-2 is detecting X-ray flashes (XRFs),
which are similar to regular “classical” GRBs in
many ways except that XRFs have larger fluence in
the X-ray band (2—30 keV) than in the gamma-rayband (30—400 keV). XRFs have receivedincreasing attention in the past several years [10].
Using twelve BeppoSAX GRBs with measured
redshifts, Amati et al. [11] showed that the spectral
peak energy at the source frame Epeaksrc and theisotropic-equivalent radiated energy Eiso are tightly
correlated, and follows a relation Epeaksrc ∝ Eiso1/2.
With the 10 HETE GRBs/XRFs with measured
redshifts we have confirmed this relation.
Furthermore, we extended this relation by threeorders of magnitude in Eiso [12]. These results
provide strong evidence that XRFs and ordinary
GRBs form a continuum, and are a single
phenomenon. The extended Amati et al relation(Epeaksrc ∝ Eiso1/2) suggests that the Epeaksrc and Eiso
are controlled by some single parameter, which
differentiate XRFs and GRBs. Understanding thiskey parameter should certainly lead to theunderstanding of the energetics and radiation
mechanism of GRBs.
Observationally it is essential to obtain larger
sample of XRFs and GRBs with measured redshifts.
In particular, additional redshift determinations areclearly needed for XRFs with 1 keV <Epeak<30 keV
in order to confirm these results and to test thetheories.
5. Coming Observations
In November 2004, Swift, a larger satellite
dedicated for the GRB study [13] was successfullylaunched, and now in the process of performanceverification. Swift is equipped with three main
instruments: Burst Alert Telescpe (BAT) for
detecting and localizing GRBs in the 15—200 keVenergy band, XRT (X-ray telescope) and UVOT(UV Optical Telescope) for imaging X-ray andUV/optical afterglows. Once BAT catches a GRBand localize it, the entire spacecraft isautonomously slewed to the location of GRB toperform the observations of the afterglow with XRTand UVOT. HETE-2 is complementary to Swiftwith the soft X-ray response. The scientificdiscoveries that HETE-2 has made need to be
confirmed and proceeded with further operation ofHETE-2 in synergy with Swift in the coming years.
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